<p>In this paper, the effects of Bi content (0–15&#xa0;wt%) on the microstructure, <i>γ</i>-ray shielding properties and energy deposition distribution of the (Sn-0.7Cu)-xBi alloys were systematically investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and Geant4 Monte Carlo simulations were used to analyse the microstructural evolution and shielding performance of the alloys. The results demonstrate Bi phase transitions: dispersed particulates → elongated/acicular phases → island-like structures → continuous Bi-rich domains. The addition of Bi increases the mass attenuation coefficient (MAC) and the effective atomic number (<i>Z</i>_eff), while reducing the half-value layer (HVL), the mean free path (MFP) and the transmission. It also shortens the radiation path length and improves <i>γ</i>-ray attenuation at an identical thickness. Energy deposition reveals three distinct regimes: low-energy <i>γ</i>-ray concentrates on the surface with high efficiency, medium-energy <i>γ</i>-ray exhibits extended penetration with volumetric deposition, and high-energy <i>γ</i>-ray achieves deep penetration with lower efficiency. Adding Bi significantly improves shielding in the low-to-medium-energy regimes. This work provides fundamental insights into designing Pb-free Sn–Cu–Bi radiation shielding materials.</p> Graphical abstract <p></p>